[Dune] same ID for several interfaces!

[Oliver Sander]

Hi Benjamin!
Thanks for reporting this.  So you claim that you have come across a
UGGrid configuration with non-unique edge indices?  That would indeed
be a bug.

However, you know that the line

grid.globalIdSet().subId(*is.inside(), is.indexInOutside(), 1);

is not what you want: you would want is is.indexInInside().  That may
already explain your problem with the double id: if you are on 38 then
indexInOutside is one of 0,1,2,3,4.  However element 38 has only
four edges.  Hence if indexInOutside happens to be 4 you run into
undefined behaviour.

Here's my suggestion:
Try the same with Dune 2.1 and see if the problem persists.
If it does persist create a test case that triggers the problem
while using the grid API correctly.  If you have such a test
case please report back to me and I will try to fix it.

Thanks in advance,
Oliver


Am 08.06.2011 13:08, schrieb Benjamin Faigle:
> Hello Dune,
>
> I maybe stumbled over some strange behaviour in dune (stable 2.0), and I
> was told to send it to the list:
>
> After some work on refinement, I had the situation where I got the same
> ID for completely different interfaces when simulating in 2D with UGgrid
> (numbers are CellIndice). I encountered a situation like this:
>
> +-------+
> | |
> | 7 |
> | |
> +---+---+
> | 39| 38|
> +---+---+
> | 36| 37|
> +---+---+
> | |
> | 6 |
> | |
> +-------+
>
> If I regard the Interface (IF) between 38 and 7 from Element 38, I
> should get an unique ID with
> grid.globalIdSet().subId(*is.inside(), is.indexInOutside(), 1);
> Works most often.
>
> With the right geometries, after many refinement steps, I had now the
> situation where this method gave the ID 798 - the same ID as for the IF
> between 36 and 6 (as seen from 36)!
>
> Of course I could fix this by using (inside, indexInInside,1), but I
> nevertheless thought that IDs should be unique.
>
> In case you are interested, tell me what information you need,
> best regards,
>
> Benjamin
>
>
> On 05/16/2011 12:04 PM, Jö Fahlke wrote:
>> Am Mon, 16. May 2011, 11:27:09 +0200 schrieb Benjamin Faigle:
>>> if i understood it right, the method
>>> grid.localIdSet().id(const EntityType&e)
>>> should return a unique ID for every sort of entities, be it
>>> elements, vertices or intersections. It works out pretty well with
>>> elements, as shown for example in the Dune Grid Howto
>>> IdType idf = idset.id (* ep );
>>>
>>> However, if I try the same with an *intersection, I get the error
>>> message:
>>> ...dune-grid/dune/grid/common/indexidset.hh:394:43: error:
>>> ‘codimension’ is not a member of ‘Dune::IntersectionIterator<const
>>> Dune::UGGrid<2>, Dune::UGGridLeafIntersectionIterator,
>>> Dune::UGGridLeafIntersection>’
>>>
>>> I am using dune 2.0.
>>> I am sorry if it is just an incredibly silly question, and many
>>> thanks for your help,
>>
>> Only entities have ids. Intersections are not entities!
>>
>> All grids have entities of codimension dim (vertices) and codimension 0
>> (elements), and all have intersections. Some grids have entities of other
>> codimensions as well. Even if a grid has entities of codimension 1,
>> these are
>> not the same as entities. Most obvious differences:
>>
>> * Intersections know the neighbouring cells, codim 1 entities don't.
>>
>> * Intersections have an orientation, codim 1 entities don't.
>>
>> * Codim 1 entities have indices and ids and can be used to obtain array
>> indices from a mapper (even if the grid does not support codim 1 entity
>> objects!) For intersections, indices, id's and maps are not provided by
>> core Dune, though it is possibly to build such things yourself.
>>
>> * In non-conforming grids you may have the following situation:
>>
>> +-------+---+---+
>> | | b | d |
>> | a +---+---+
>> | | c | e |
>> +-------+---+---+
>>
>> Elements b, c, d and e all have four codim 1 subentities and four
>> corresponding intersections. Element a also has four codim 1 subentities,
>> but it has five intersections. The codim 1 entity to the right of b is
>> the
>> same as the codim 1 entity to the left of d. You have three codim 1
>> entities to the right of a however: on covering the whole of a's right
>> border, and two smaller one each covering the upper and the lower half of
>> a's border, respectively.
>>
>> If you know you have a conforming grid, you have a correspondance between
>> intersections and codim 1 entities (ignoring the orientation of the
>> intersections). In that case you can usually get away by using the id
>> of the
>> corresponding codim 1 entity as the id of the intersection (even if
>> the grid
>> does not support codim 1 entities as independent objects. Use
>>
>> grid.globalIdSet().subId(e, i.indexInInside(), 1);
>>
>> where i is the intersection and e is the codim 0 entity i was obtained
>> from.
>> If you don't have e readily available, you can use
>>
>> grid.globalIdSet().subId(*i.inside(), i.indexInInside(), 1);
>>
>> instead.
>>
>> Bye,
>> Jö.
>>
>
>
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